Semester 3 Nutrition - Class 11 West Bengal Board Zoology

Human Nutrition and Metabolism

Nutrition involves the intake, digestion, absorption, and metabolism of food substances to provide energy and maintain body functions. This summary covers key concepts, enzyme mechanisms, metabolic pathways, and food safety relevant to human nutrition, as explicitly discussed.

Enzyme Inhibition and Regulation

Enzyme activity can be regulated by various inhibitors and feedback mechanisms. Feedback inhibition occurs when the concentration of the product increases, leading to inhibition of the enzyme's activity.

• Competitive Inhibitors: Bind temporarily to the active site; example: Ritonavir.
• Non-Competitive Inhibitors: Bind to the allosteric site, not the active site; example: Cyanide.…

Human Nutrition and Metabolism

Nutrition involves the intake, digestion, absorption, and metabolism of food substances to provide energy and maintain body functions. This summary covers key concepts, enzyme mechanisms, metabolic pathways, and food safety relevant to human nutrition, as explicitly discussed.

Enzyme Inhibition and Regulation

Enzyme activity can be regulated by various inhibitors and feedback mechanisms. Feedback inhibition occurs when the concentration of the product increases, leading to inhibition of the enzyme's activity.

• Competitive Inhibitors: Bind temporarily to the active site; example: Ritonavir.
• Non-Competitive Inhibitors: Bind to the allosteric site, not the active site; example: Cyanide.
• Irreversible Inhibitors: Bind permanently, causing lasting inhibition; example: Penicillin.
• Allosterism: Binding at a site other than the active site alters enzyme activity, which can be positive or negative.
• Turnover Number (Catal): Number of substrate molecules converted per second by one mole of enzyme; also called "catal" or "kat".
• Michaelis Constant ($K_m$): Indicates the substrate concentration at which the reaction rate is half of its maximum.

Enzyme Structure and Components

Enzymes are often composed of protein and non-protein parts, each with specific terminology.

• Apoenzyme: The protein part of an enzyme.
• Cofactor: The non-protein part, which may be:
  • Coenzyme: Loosely and temporarily attached (e.g., NAD).
  • Prosthetic Group: Firmly and permanently attached.
• Holoenzyme: The complete, active enzyme with both apoenzyme and cofactor.
• Proenzyme (Zymogen): Inactive precursor of an enzyme (e.g., pepsinogen for pepsin).
• Isoenzymes: Different enzymes with similar functions but different amino acid sequences.

Digestion of Carbohydrates

Carbohydrate digestion begins in the mouth and continues in the small intestine, involving specific enzymes and optimal conditions.

• Salivary Amylase (Ptyalin): Acts on cooked starch in the mouth; optimal pH 6.5, optimal temperature 45°C; found in saliva; activated by chloride and bromide ions.
• Maltase: Converts maltose to two glucose molecules.
• Pancreatic Amylase: Acts on both cooked and uncooked starch in the small intestine.
• Carbohydrate digestion starts in the mouth and is selective in the stomach (only sucrose is digested due to HCl).
• Infants under 6 months: Should not be given carbohydrate-rich foods as pancreatic amylase is not yet produced.

Absorption and Transport of Nutrients

Absorption mainly occurs in the small intestine, with specific mechanisms for different nutrients.

• Villi: Main site of absorption in the small intestine.
• Glucose and Galactose: Absorbed via sodium-dependent co-transport (symport) using SGLT transporters.
• Fructose: Absorbed via sodium-independent facilitated diffusion (GLUT5).
• Floridzin: Inhibits glucose-sodium co-transport.
• Monosaccharides: Fructose is converted to glucose in the liver after absorption.
• Fat Absorption: Involves micelle and chylomicron formation; micelles are formed from monoglycerides, fatty acids, cholesterol, bile salts, sodium carbonate, and vitamin D; chylomicrons contain phospholipids, triglycerides, cholesterol, and protein.

Metabolic Pathways and Energy Production

Metabolism includes both anabolic (building) and catabolic (breaking down) processes, with specific cycles and pathways.

• Anabolism (Biosynthesis): Increases dry weight and stores energy (e.g., glycogenesis).
• Catabolism (Degradation): Decreases dry weight and releases energy (e.g., glycolysis, glycogenolysis).
• Glycolysis (EMP Pathway): Occurs in cytoplasm; 10 steps; produces 7 ATP (modern view); substrate is glucose, end product is pyruvic acid.
• Krebs Cycle (TCA/Citric Acid Cycle): Occurs in mitochondria; produces 20 ATP per glucose (modern view); first product is citric acid.
• Link Reaction: Pyruvic acid is converted to acetyl-CoA before entering the Krebs cycle.
• Cori Cycle (Lactic Acid Cycle): Connects muscle and liver; lactate from muscle is converted to glucose in the liver; involves insulin, glucagon, and adrenaline.
• Glycogenesis: Formation of glycogen from glucose (stimulated by insulin).
• Glycogenolysis: Breakdown of glycogen to glucose (stimulated by glucagon).
• Gluconeogenesis: Formation of glucose from non-carbohydrate sources (amino acids, fatty acids, lactic acid).

Protein and Nitrogen Metabolism

Protein metabolism involves deamination, transamination, and urea formation, with specific pathways and excretory products.

• Amino Acid Pool: Total amino acids in the body and from diet.
• Deamination: Removal of amino group, producing ammonia (toxic), which is converted to urea in the urea cycle (occurs in liver cytoplasm and mitochondria).
• Transamination: Transfer of amino group between amino acid and keto acid; occurs in liver, kidney, brain, heart, pancreas.
• Decarboxylation: Removal of carboxyl group; occurs in cytoplasm.
• Transmethylation: Transfer of methyl group; occurs in liver and other cells.
• Urea: Normal blood level 25–30 mg/dL.
• Excretory Types:
  • Ureotelic: Humans, sharks.
  • Uricotelic: Insects, reptiles, birds.
  • Ammonotelic: Aquatic vertebrates.

Food Energy, Calorimetry, and Measurement

Energy content of food is measured in calories and joules, with specific methods and units.

• Calorie: Amount of heat to raise 1 g water by 1°C.
• Kilocalorie (kcal): 1 kcal = 1000 calories.
• Joule: SI unit of energy; 1 kcal = 4.2 kJ.
• Bomb Calorimeter: Measures gross energy (direct method).
• Benedict-Roth Apparatus, Douglas Bag: Indirect calorimetry.
• Energy Values:
  • Carbohydrate: 4 kcal/g
  • Protein: 4 kcal/g
  • Fat: 9 kcal/g
• Basal Metabolic Rate (BMR): Calculated using Harris-Benedict formula; depends on weight (kg), height (cm), age (years).
• Body Mass Index (BMI): $BMI = \frac{\text{weight (kg)}}{[\text{height (m)}]^2}$
• Total Energy Expenditure: $TEE = BMR \times$ Physical Activity Level (PAL)
• Respiratory Quotient (RQ): Ratio of CO$_2$ produced to O$_2$ consumed; RQ for carbohydrate = 1, protein = 0.8, fat = 0.7.

Food Safety, Microbiology, and Preservation

Food spoilage and safety depend on microbial activity, preservation methods, and regulatory standards.

• Types of Food Based on Water Content:
  • Perishable: High water content (e.g., fish, meat).
  • Semi-perishable: Moderate water content (e.g., potatoes, onions).
  • Non-perishable: Low water content (e.g., flour, grains).
• Microbial Types:
  • Psychrophilic: Survive at 0°C or below.
  • Mesophilic: 20–45°C (e.g., Thiobacillus).
  • Thermophilic: Above 45°C.
  • Acidophiles: Survive in acidic environments.
  • Alkaliphiles: Survive in alkaline environments.
• Food Fermentation:
  • Lactobacillus: Converts milk to curd.
  • Streptococcus faecalis: Used in idli/dosa fermentation.
  • Saccharomyces cerevisiae: Used in wine and bread making; called brewer's or baker's yeast.
  • Aspergillus oryzae: Used in soy sauce production.
• Foodborne Diseases:
  • Giardiasis: Caused by Giardia lamblia.
  • Amoebiasis: Caused by Entamoeba histolytica.
  • Cholera: Caused by Vibrio cholerae.
  • Typhoid: Associated with rose spots.
  • Staphylococcal Food Poisoning: Caused by Staphylococcus, produces enterotoxin.
  • Aflatoxin: Produced by Aspergillus, contaminates grains.
• Preservation Methods:
  • Blanching: Inactivates enzymes (catalase, peroxidase) in vegetables by heating at 94–100°C for 2–5 minutes, then cooling rapidly.
  • Browning: Caused by phenol oxidase in cut fruits.
• Cleaning Agents:
  • Sodium Hydroxide, Sodium Carbonate: Alkaline cleaners.
  • Tartaric Acid: Organic acid cleaner.
  • Phosphoric Acid: Inorganic acid cleaner.

Food Standards and Regulatory Bodies

Food safety and quality are maintained through specific standards and organizations.

• HACCP: Hazard Analysis Critical Control Point; aims to prevent foodborne illnesses.
• FSSAI: Food Safety and Standards Authority of India; headquarters in New Delhi.
• Codex Alimentarius: Established by FAO and WHO; headquarters in Rome.
• Pasteurization: Named after Louis Pasteur, who demonstrated microbial spoilage in food.

Special Topics and Exam Traps

Certain concepts and exceptions are frequently tested and should be noted.

• Diabetes Mellitus:
  • Type 1: Juvenile, due to lack of insulin production.
  • Type 2: Adult, due to insulin resistance.
  • Gestational Diabetes: Occurs during pregnancy.
  • Symptoms: Polyuria, polydipsia, polyphagia ("three Ps").
  • Renal Threshold for Glucose: 180 mg/dL.
• Blanching: Inactivates most enzymes except pectin methyl esterase, which may remain active.
• BMR Influences:
  • Decreases during sleep (5%).
  • Increases with fear, cancer, pregnancy (especially 2nd and 3rd trimesters), fever, hyperthyroidism, and after quitting smoking.
• Exam Pitfalls:
  • Confusing enzyme types and their examples.
  • Misidentifying absorption mechanisms for sugars.
  • Overlooking the difference between perishable, semi-perishable, and non-perishable foods.
  • Not distinguishing between direct and indirect calorimetry methods.
  • Forgetting the specific temperature and pH optima for enzymes and microbes.